Sebastian Vettel blitzed the field once again in second practice for Formula 1's United States return at Austin, despite sitting out the majority of the session with a water leak on his Red Bull.
The World Champion had to park in the garage after just three laps at the start of the afternoon when the team detected the water system fault. Vettel would not reappear for almost an hour, but when he did, he left his rivals in no doubt whatsoever about his pace this weekend as a 1m37.718s lap on his second medium-tire run put him 0.757sec clear of the pack.
Even during Vettel's absence, Red Bull had been in front for most of the session thanks to Mark Webber. The Australian had produced a 1m39.485s on hard tires just after the half-hour mark to depose early pacesetter Nico Rosberg's Mercedes, and then found another two tenths next time round.
Felipe Massa enjoyed a seven-minute spell in first place when he delivered a 1m39.106s, but soon after Webber switched to the softer Pirellis and regained the top spot.
Webber's first medium-tire effort was a 1m38.958s, and he swiftly improved to 1m38.475s - although that was firmly eclipsed by Vettel's pace when the German was back in action.
There was some cause for optimism for Vettel's title rival Fernando Alonso's Ferrari team. Alonso took third, just 0.008sec down on Webber, and Massa had at least ended up quickest in the 'hard tire' segment of the session.
The McLarens completed the top five, Lewis Hamilton overcoming a Turn 13 spin to top teammate Jenson Button. Sometime pacesetters Massa and Rosberg ended up sixth and seventh.
Bruno Senna was top Williams driver in eighth, while his teammate Pastor Maldonado suffered an apparent engine problem in the closing seconds.
Kamui Kobayashi lost a lot of track time to an exhaust problem before taking his Sauber to ninth ahead of Michael Schumacher's Mercedes.
The Lotus duo were just outside the top 10. Romain Grosjean tested the limits of the dramatic Turn 1 with a quick spin late in the session, emulating an earlier error by Caterham driver Vitaly Petrov.
The other Caterham also had an incident. Heikki Kovalainen had to nurse his car around a full lap with a puncture following a light brush with Jean-Eric Vergne's Toro Rosso, which sustained front wing damage in the tangle.
At the tail of the field, the troubled HRT team did not leave the garage for the first half of the session and completed significantly less mileage than most of the pack.
Pos Driver Team Time Laps
1. Sebastian Vettel Red Bull-Renault 1m37.718 17
2. Mark Webber Red Bull-Renault 1m38.475s + 0.757 36
3. Fernando Alonso Ferrari 1m38.483s + 0.765 35
4. Lewis Hamilton McLaren-Mercedes 1m38.748s + 1.030 31
5. Jenson Button McLaren-Mercedes 1m38.786s + 1.068 32
6. Felipe Massa Ferrari 1m39.029s + 1.311 33
7. Nico Rosberg Mercedes 1m39.448s + 1.730 32
8. Bruno Senna Williams-Renault 1m39.531s + 1.813 38
9. Kamui Kobayashi Sauber-Ferrari 1m39.653s + 1.935 36
10. Michael Schumacher Mercedes 1m40.115s + 2.397 30
11. Kimi Raikkonen Lotus-Renault 1m40.166s + 2.448 32
12. Pastor Maldonado Williams-Renault 1m40.230s + 2.512 35
13. Romain Grosjean Lotus-Renault 1m40.2$6s + 2.568 32
14. Sergio Perez Sauber-Ferrari 1m40.326s + 2.608 35
15. Daniel Ricciardo Toro Rosso-Ferrari 1m40.435s + 2.717 30
16. Jean-Eric Vergne Toro Rosso-Ferrari 1m40.516s + 2.798 32
17. Nico Hulkenberg Force India-Mercedes 1m40.700s + 2.982 34
18. Paul di Resta Force India-Mercedes 1m41.430s + 3.712 35
19. Heikki Kovalainen Caterham-Renault 1m42.476s + 4.758 38
20. Timo Glock Marussia-Cosworth 1m42.652s + 4.934 33
21. Vitaly Petrov Caterham-Renault 1m42.846s + 5.128 38
22. Charles Pic Marussia-Cosworth 1m43.538s + 5.820 38
23. Pedro de la Rosa HRT-Cosworth 1m44.453s + 6.735 16
24. Narain Karthikeyan HRT-Cosworth 1m45.114s + 7.396 20